The air cushion masks are designed to increase the seal pressure as the air pressure behind it increases. Nice design. Then they put a flexible headgear on the mask so when the pressure increases the flexible material in the headgear stretches more until guess what - the seal blows out due to insufficient contact area. I guess that part of the rationale is the relaxation of the tissue during sleep requires something that will adapt to that and, of course, user comfort for people that just do not want to feel too much tightness on their face or are sensitive to it. It makes me think that is part of the reason for so much difference in personal performance of the different masks.

I know my Mirage Quattro, is tight when I put it on at night but feels a lot looser when I get up in the morning. My leak rates are generally very good all night long unless I have seriously screwed something up.

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